The sun was just dipping below the horizon and the earliest stars were shining as Ben ran for the school's tennis court, desperately hoping he'd make it. But once he reached the bleachers, he knew he was too late. The stands were mostly empty, except for a few friends and family members congratulating the players. Julie was standing with her parents, her racket zipped up in its case. The moment her father caught sight of Ben, his face went grim, and Ben couldn't help but wince. Julie turned around to see him, and disappointment was written all over her face.

"You're late," she said flatly as he walked over to her.

"I know," he answered. "And I'm sorry. But there was this alien attacking Chicago—a Rath, and you know how hard he is to handle." The look Julie was giving him was dangerously close to a glare, and his face fell. "It was an emergency."

"That's what you said last time," she pointed out, looking down.

Neither one could look at each other, and Julie's parents decided to give them some time alone. For a long time, Ben didn't know what to say, how to apologize. How exactly could he explain that he was trying his hardest, that he'd wanted to be there but he couldn't exactly tell the Plumbers no? That when he tried to speed up the fight by going Ultimate, Gwen told him he was overdoing it and he'd only put more people in danger?

"I'm sorry," he tried again.

"I barely see you anymore," she said. "You never have time for me."

"That's not true," Ben protested. "I…"

"There's always something," Julie insisted. "You come over to my house, and an alien attacks. We try to have lunch together, but the press chases you off. And that's when we actually plan something. Otherwise, you're running around all over the galaxy."

"I can't help it," Ben said. "I can't just drop everything. They need me—it's my responsibility."

Was that a tear that had fallen on the ground in front of Julie, or was it his imagination? Her voice sounded steady as she said, "I know. But you have a responsibility to me too."

"I…" Ben tried, but he couldn't figure out the right thing to say.

"Considering how little time you have for me lately, we might as well be broken up."

Ben looked at her in shock, feeling the world shatter. She'd promised she'd always be there for him. That no matter what troubles his fame brought him, she still loved him. This couldn't be happening. Not after everything they went through—they'd been there for each other all this time, through the absolute worst, and she was breaking up with him?

"Ben!"

Gwen's voice cut through his shock, and he turned around to see her and Kevin running toward him.

"You're not answering your phone or the Ultimatrix," Gwen scolded.

"The Plumbers just called," Kevin explained. "Havoc escaped. He's in Tokyo right now, so I hope you like sushi."

Ben didn't know what to do. The team needed him right now, but Julie…

"Go," she said softly behind him. He turned around to look at her and saw that her eyes were closed, without a trace of tears.

Pain turned into anger, and he swept up the pieces of his broken heart as he coldly answered, "Fine. If that's what you want, I'm gone."

Before anyone could stop him, he turned and ran away, making a beeline for the Rustbucket, for Tokyo, for anywhere that was away from here. It would be afternoon in Japan, so the stars in the sky right now wouldn't be mocking him.

He didn't notice one of the stars drop from the sky and enter him. He just assumed it was the feeling of the knife in his back twisting. Nor did he hear the voice whispering to him or see the head rising out of the sand. He just ran through it, kicking up a sandstorm as he tried to run away from it all.

And somewhere in time, a demonic-looking creature in red stopped mid-argument and looked out the window of a speeding train.

"I smell an Imagin."

"Eh?" asked one of his human companions. "Are you sure?"

"Of course I'm sure! I'd know that smell anywhere! Let's go!"

Derailed
Chapter One: "What Did Butterflies Ever Do To You?"

One pair of hands caught the clawed fist heading directly to his stomach while the other punched Havoc in the jaw. Havoc reeled but soon recovered, roaring at Four-Arms. He brought in both fists, claws out, toward Four-Arms's head, but a pink energy shield formed just in time to block it.

"Focus, Ben," Gwen warned. "You can't afford to lose your temper out here!"

"I'm fine," he growled between clenched teeth. He glared at her, and she decided against arguing with him, opting to rejoin the battle.

Kevin had absorbed the pavement and had his hands morphed into sledgehammers, swinging wildly at Havoc. Normally, an all-out melee battle would be dangerous, but Japanese civilians were good about keeping far away from attacks. It made Ben feel better about going all-out himself, and he reached for the Ultimatrix.

"Tell me your wish."

Ben stopped suddenly, glancing around. The voice almost sounded familiar, but it wasn't Gwen or Kevin, and nobody else was around either.

He shook his head, muttering, "Great. Now I'm hearing things. This is perfect."

He set his hand on the Ultimatrix and changed forms, shouting, "Humongousaur!" a second later. Tough plates formed along his back and head as extra protection as he charged Havoc, plowing through sand along the way. Immediately, Gwen and Kevin got out of the way as Humongousaur rammed into Havoc, sending him flying across the street and into the front door of a café.

"Ben!" Gwen shouted. "We're not here to destroy the city!"

"You got a better way to take him down?" Humongousaur challenged. "Remember what he did in Chicago?"

"Hey, stop taking this out on us," Kevin argued, ignoring the look of horror and then the growl of anger Humongousaur gave him in response. "Let's take this guy in already."

"Fine by me," Humongousaur growled, growing to his maximum height. He picked up Havoc and held him over his head for a moment before slamming him toward the ground.

"I will grant any wish."

The voice came again, but disappeared with a burst of sand from Humongousaur's attack. The ground fractured from the impact, with sand slipping down the cracks in the earth. As Havoc lay unconscious on the ground, for all intents and purposes harmless, Ben transformed back to human.

"There," he said, giving his friends a hard look. "He's down."

"You really need to tone it down with the attitude," Gwen replied, crossing her arms.

"Hey!" Ben shouted, his temper flaring. "You try going through what I did today! Then you can tell me—"

While he was in the middle of his rant, Havoc sprang to his feet with a roar of fury. Ben had no time to react as Havoc struck him across the face, dropping him to the ground. He was faintly aware of Gwen and Kevin calling his name, but his consciousness was slipping fast.

He could make out one more voice, however—the unknown whisper in his mind:

"Just pay me one thing."

There was sand in his eyes. Ben squeezed his eyes tighter, but the grit in his eyes wouldn't relent.

"Oh? I think he's waking up."

The voice wasn't familiar, so out of instinct, he opened his eyes and tried to sit up, but sharp pain shot through the side of his head. Gentle hands helped him into a sitting position as he clamped a hand over one of his temples.

"Thank you, Ms. Nogami," Gwen said as she came over to Ben's side. The woman that had helped Ben smiled at them and got up, taking a bowl of water and a cloth away with her.

Ben took a moment to look around, realizing he was in some kind of café. The smell of coffee came from the counter, where the woman accepted a cup from Kevin and took it to the sink. All around the café were books on stars and astronomy memorabilia—the most obvious an antique telescope on display.

"What…" Ben started.

"This is Ms. Nogami's coffee shop," Gwen explained. "She told us to get you inside when you got hit, and she's been keeping an eye on you since. It's especially nice, considering we smashed her front door."

"It's no problem," Ms. Nogami insisted. "These things happen."

"Thanks," Ben said to her, and she smiled in return. "What happened to Havoc?"

"Backup finally showed," Kevin said, walking over. "Weirdest bunch of Plumbers I ever saw, though."

"I don't know if they were Plumbers," Gwen pointed out.

Kevin shrugged. "They had a badge and managed to call in a transport ship. Maybe they were the SWAT team?"

Ben stood up, feeling a wave of dizziness the moment he started to get to his feet. He swayed dangerously, but Gwen and Kevin held him steady.

"You need to take it easy," Gwen warned. "Havoc hit you pretty hard. We couldn't get you to wake up."

Bitterness and heartbreak surged within him, and Ben couldn't help but mutter, "Maybe I shouldn't have."

Gwen sighed, and the look Kevin gave him was so unusually sympathetic that Ben couldn't take it anymore. Offering a last thank you to Ms. Nogami, he slowly walked out the broken door.

"Ben, wait up!" Gwen called as they followed him out. "We need to talk."

"No, we don't," Ben said flatly.

Kevin grabbed his shoulder and forcefully turned him around. Ben flinched in pain, but he met Kevin's stare evenly as his friend insisted, "Now, Ben."

"We heard what Julie said," Gwen admitted.

Ben pulled himself away from Kevin and started to walk off. "Then you know everything you have to."

"Listen!" Gwen cried. "We're sorry about what happened, but I don't think she was saying what you heard. She's just frustrated…"

Ben came to a stop and turned back to glare at her. Grains of sand started to drop off of his body and onto the ground, but it was such a small amount that nobody noticed. "What, that I'm only there for her whenever she needs me? I did everything I could to be there for her games and everything else. I can't help it if an alien's attacking Chicago or Tokyo at the same time she's in Bellwood. She knows that! And even when I am there, I can't show my face without having a camera and microphone shoved at me. I'm trying."

"We know you are," Gwen insisted.

"And you know what's worse?" he added. "She said she'd be there for me. When all of this went down and my secret got out, she said she was still there for me. Now all of a sudden, when it's inconvenient for her, she dumps me?"

"Nobody's saying your life's exactly roses," Kevin said. "But right now, you need to cool down and stop worrying about her for a minute…"

"Stop worrying?" Ben shouted. "You try having the one person who promised she'd always be there just drop you out of nowhere! And why? Because your life is completely out of your control. I never wanted any of this—not the cameras, not the stalkers, not the pundits turning me into Public Menace Number One!"

The sand was pouring off of him in greater amounts now, collecting in a heap on the ground. Ben was too angry to notice, but Gwen tugged on Kevin's arm and pointed.

"You gotta calm down," Kevin insisted.

"How can I calm down?" Ben demanded. "I just want everyone to forget I even existed. You know what? Maybe I just wish I never existed at all. Maybe that would fix things."

The sand stopped flowing, leaving a pile on the ground around Ben. The pile of sand then coalesced into a head and upper body, with a pair of legs made of sand hovering in the air over it. The legs lowered to the ground and the head rose until a whole form appeared and solidified.

It was a girl, or as close to the concept as possible. Her short hair was made of green vines, rising into a bun like a pumpkin's stem. Her skin was pale orange, much paler than the short, pumpkin-shaped dress she wore. Vines ran down her arms and legs, and on her feet were green glass slippers. But it was her face that startled everybody.

"She looks just like Julie," Gwen gasped.

The Cinderella-Julie creature turned to Ben and held out her hand, as if asking him to dance.

"Your wish shall be granted," she said in Julie's voice.

The next thing anyone knew, a bright green line formed down Ben's body, splitting him in half. The two halves opened, revealing a swirling grey vortex, and the creature flew within.

"Ben!"

Gwen and Kevin shouted together as they ran toward Ben, whose body sealed itself up once the creature had gone. He staggered backwards and fell, an expression of numb terror on his face. As they knelt next to him, Gwen wasted no time trying to sense the creature's presence.

"Find anything?" Kevin asked anxiously.

"No," Gwen said, her eyes glowing. "I can't sense it anywhere inside him! Whatever it's doing, it's keeping too far out of my reach."

All at once, Ben's life seemed to flash before his eyes. Time went backward from that point, and in his mind, he heard the chimes of the midnight bell: one, two, three...

"Something's wrong," he murmured in a daze.

"Yeah, no kidding!" Kevin answered.

Ben shook his head. "She's in my memories. I don't know what she's doing, but I can hear the bells." Four, five, six... "When they reach twelve, something's going to happen. They'll be gone."

Kevin looked to Gwen cautiously. "You think it's trying to brainwash him?"

"I don't know," she admitted.

"No," Ben insisted. Seven, eight, nine... "I'm not forgetting, but it's…"

"Out of the way!" shouted an unfamiliar voice as someone pushed his way over. Ben had only a glimpse of spiked hair and startlingly red eyes before a train ticket obscured his view. "Come on, what's the date?" Ten, eleven...

Twelve.

All of a sudden, a bright white light washed through the landscape, making everything fade. Ben gasped in shock and got to his feet, and the other kid turned and swore as the whiteness spread, taking out Gwen and Kevin before anyone could stop it.

"Got to get back to…" the kid started.

There was a red flash from the kid's body, like something was leaving it, but soon, the nothingness caught up with him too. He disappeared, leaving Ben alone in the blankness, staring up as faint, rainbow colors slowly spread across the sky.

And suddenly, it was a sky again—iridescent, yes, but still recognizably a sky. The blankness was gone, and Ben was standing in an unfamiliar desert under an unknown sky.

"What?" he asked. "Where…"

"Ben Tennyson."

Ben turned around to see a stern-looking Asian man with a cane standing in front of a bullet train. Though Ben looked around, he didn't see any sign of civilization, which made the train appear out-of-place. Even stranger, there didn't appear to be any tracks ahead of it, and only a few behind.

"What's going on?" he asked.

But the man continued to look serious, only answering, "DenLiner awaits."

The man turned and walked inside the train, and having no other choice, Ben followed him. The man led him past the junction between cars and into a dining car, where a red, demonic-looking creature sat moodily at a table, shooting him an odd glare. Ben wondered briefly if he'd done anything to annoy this alien, but before he could ask, a perky young woman with a shock of hot pink in her hair jumped into his line of sight.

"Hi!" she greeted. "Welcome to DenLiner. Coffee?"

Without even waiting for an answer, she thrust a cup at him, pointing him to a table. It was just to the side of where the red alien was, giving him a perfect vantage point to shoot Ben another death glare. Trying to ignore him, Ben sat down and started to take a sip of his coffee. He stopped quickly when he realized it was covered in what looked distinctly like cake icing. He liked sugar as much as the next person, but this seemed to be pushing it.

"What's wrong?" the alien asked critically. "Coffee not good enough for you?"

Ben set down the cup, trying for his most neutral tone. "I'm not much of a coffee drinker."

"Momotaros-kun, that's enough," the man said, walking over. "Naomi-kun, please close down the dining car."

"Okay!" the server answered cheerfully.

As Naomi began calling the announcement into a phone, the man fixed Ben with a critical look. Between the man analyzing him and the creature glaring at him, he was really beginning to feel uncomfortable. Unable to take it any longer, he burst out, "What's going on? Where are my friends? And what is this place?"

To his surprise, the man smiled in amusement. "Paradox-hakase said you would be full of questions."

"Wait, Paradox?" Ben asked, ignoring the unfamiliar word. "You know Paradox?"

The man nodded. "He also wanders the path of time, though through a different method than we use."

He paused, as if for dramatic effect, and Ben waited for him to continue. When he didn't, he asked, "And you use?"

"Tch," the creature, Momotaros, replied. "And I thought Ryotaro took a while to catch on. DenLiner is a time train—we're traveling through time right now!"

Ben stared at Momotaros in disbelief. "You're kidding."

"No," the man answered. "DenLiner, along with other time-trains, travels the rails of time. Only those with a ticket or a pass are allowed to travel through time. Which reminds me." He reached into the breast pocket of his coat and removed a small, black metal pass. "Paradox-hakase asked me to give this to you."

"A Rider Pass?" Momotaros asked angrily, jumping to his feet. "Owner, how can you give it to this kid? What about Ryotaro?"

"You're getting ahead of yourself, Momotaros-kun," Owner scolded. "There is still much that Ben-kun needs to know."

Owner gave Momotaros a heavy look until he calmed and sat back down. Ben silently watched Momotaros's explosion, but he couldn't help but feel a bit of sympathy. Whatever had happened to his friend, Ben couldn't help but feel the same worry about his own friends.

"What happened to my friends?"

Without betraying emotion, Owner asked, "Has Paradox-hakase explained to you what a singular point is?" When Ben shook his head, he continued, "A singular point is a person who is unaffected by changes in the timeline. They can exist outside of time, and their memories remain constant, no matter what happens to their pasts. Like Momotaros-kun's friend, Ryotaro-kun, you too are a singular point."

Ben slowly nodded, remembering how Paradox had explained to the younger version of him at the Forge of Creation that his memories wouldn't be entirely gone. "Okay, so what's so important about a singular point?"

"Feh," Momotaros commented. "Only a singular point can become Den-O."

"Kamen Rider Den-O is the protector of the timestream, a warrior for DenLiner," Owner added. "There is only one person known who can become a Kamen Rider without being a singular point, but he is a special case."

"So because I remember what originally happens in the timeline, I can help keep it all straight?" Ben guessed, eliciting a nod from Owner. "But what's threatening it?"

Owner's answer was simple and mysterious: "The Imagin."

Ben had the feeling that Owner was going for a deep, enigmatic atmosphere. Unfortunately, he didn't have much patience for it. "Ima what?"

"Imagin," Momotaros answered. "Like me—we possess people, grant wishes, and destroy the timeline."

Ben gave Momotaros a look of shock before Owner explained, "Momotaros-kun and a few others like him chose to preserve the future rather than destroy it. The Imagin are formless beings until they possess someone with a subconscious wish, taking the form of something from their imagination."

"Imagine," Ben murmured in realization. "So that thing that came out of me? Was it an Imagin?"

"Correct," Owner replied. "Normally, singular points are resistant to their temptations, but this one was particularly determined, and they can be clever. One careless comment was enough to be taken as a wish."

"But I didn't wish…" Ben started before remembering just what he'd said, and he went silent in shock.

"You mistakenly wished not to exist, for no one to remember you," Owner pointed out. "Imagin have the power to connect to a person's most precious memory and use it to travel back in time to destroy the timeline. That Imagin flew into your memories and erased you from the timeline. But because of all you had done when you were within time, the timeline you affected is now gone."

"Lucky you," Momotaros said derisively to a shell-shocked Ben. "You don't exist, but you still do."

Ben held his head in his hands, stunned. This was the biggest mistake he'd ever made. He'd always known that one little slipup could end in someone dying, and he'd had some pretty staggering odds before, but it had never been like this. He hadn't been careful, and now he'd stepped on a butterfly—a mistake big enough to destroy the timeline itself.

He knew he couldn't afford to feel sorry for himself right now. He looked back up at Owner with a steely expression, one that even took Momotaros aback. "How do I fix it?"

Owner smiled with pride, as if Ben had passed some kind of test. "Your memories hold the key. They provide a tenuous link to the timeline."

"How?" Ben asked. "It doesn't exist anymore, right?"

"Memory is a powerful thing," Owner insisted. "As long as something is remembered, it is never truly destroyed. Which is why the Imagin will come after you again."

"To finish me off?" Ben guessed.

"No, for payment," Momotaros answered. "Now that she's fulfilled the contract, your strongest memory's the last thing she needs to fly to."

"My strongest memory?" Ben repeated.

Owner flicked his hand, producing a ticket between his fingers. He flipped it around and set it against Ben's head. Seconds later, a date appeared, in June 2007. He flipped it back around to show Ben and asked, "Do you remember this date?"

"That was the last day of school six years ago," Ben said, taking the ticket in surprise. He held it gingerly, as if anything else might destroy his fragile link to the past. "It's the day Grandpa took me and Gwen on the road trip for the summer, the night I found the Omnitrix." He looked up at Owner. "Is this what the Imagin wants?"

"So it would appear," Owner answered.

"I've got to stop him," he decided, standing up. "But how?"

"Momotaros-kun?" Owner asked.

The Imagin sighed. "Take him to DenBird?"

Owner nodded. "Also, you will have to help Ben-kun just as you did Ryotaro-kun. Until such a time that the original timeline can be restored, Ben-kun must be Den-O in Ryotaro-kun's place."

Momotaros groaned angrily, sounding as though he were about to fight Owner. But after another minute or two of Owner impassively watching him, he calmed down and inclined his head toward Ben.

"Fine. This way."

Ben started to follow Momotaros out, but Owner called out, "Ben-kun," making him stop and turn.

"Uh, yes?" he asked.

"As you may have noticed, our translator has a problem with English." At this, Ben looked down at the Ultimatrix; now that he thought of it, he wondered why it hadn't overridden the malfunctioning translator the others must have been using. "If you get into any trouble, hold out your pass and call out, 'Henshin.'"

"Henshin," Ben repeated, as Momotaros looked up at the ceiling in annoyance. "Got it."

"Hey, hey!" Momotaros called, taking Ben by the shoulders and turning him around to push him through the door. "Let's go, let's go, let's go!"

Momotaros pushed Ben through the other cars, where passengers took no notice of a teenage human being pushed along by an annoyed Imagin. For all Ben knew, this could have been a normal occurrence on DenLiner. When they finally reached the front of the train, Ben saw a sport motorcycle running in place in front of the window. Every so often, the handlebar of the bike would turn, and so too would the tracks being formed in front of DenLiner.

"Whoa," he breathed. "This is DenBird? It's running the train?"

"What did you think could power something like this?" Momotaros asked. "Don't think Owner's just going to get on a bicycle?"

Ben was about to argue with him, but he held out his hand. Realizing that Momotaros needed the ticket and pass, he handed them over. Expertly, Momotaros inserted the ticket into the pass and flipped the top to the opposite side before inserting the whole thing into a slot in the bike's dashboard. The moment he did, a new rail formed in front of DenLiner, splitting off from the main line.

"Nice," Ben said, taking the handlebars.

"Hey, what do you think you're doing?" Momotaros asked, putting his hands on the bars ahead of his.

"Owner said I'd have to be Den-O for now," Ben pointed out. "Isn't this Den-O's job?"

"Feh, you're not Den-O yet," Momotaros argued, swinging a leg over the bike. "And after all you broke, you're lucky I'm letting you join in the climax in the first place."

Ben raised his eyebrow, wondering if the translator had glitched again. Momotaros's use of "climax" didn't seem to fit properly. But even if he did say anything, Momotaros wasn't listening. He was already steering the bike and train over to the new track as it lifted off the ground and rose into the air. A small vortex had formed in the sky.

"You might want to hold onto something," Momotaros said as Ben began to brace himself against the wall. "The climax starts now!"

Ben 10: Ultimate Alien is the property of Cartoon Network, and Kamen Rider Den-O is the property of Toei. The title comes from a line spoken by the Tenth Doctor in the Doctor Who episode "The Shakespeare Code":

Martha: It's like in the films—if you step on a butterfly, you change the future of the human race.

Doctor: Oh, well, then don't step on any butterflies. What have butterflies ever done to you?

To mesh the timelines of both series, I've set the original Ben 10 in 2007. While this fic was started after "The Transmogrification of Eunice" and "Eye of the Beholder," it's not so much meant to resolve the Ben/Julie breakup as it is to cope with Ben's myriad of issues with his identity being revealed throughout Ultimate Alien. But I'm going to try to steer clear of too much angst in this; it's meant to be a fun crossover. I admit to some inspiration from Eternal-Night-Ride's excellent account of the breakup, "Almost Here," though I purposely tried to go a different route so I wouldn't outright copy her.

Cover art also by Eternal-Night-Ride.